1. Field of the Invention
This invention relates to phase-locked loops and more particularly, to the optimization of acquisition time and tracking control of phase-locked loops.
2. Description of the Prior Art
Phase-locked loops (PLL) have a wide range of applications. In the field of digital transmission systems PLL's are commonly used to recover the average frequency, and hence the timing information, of discontinuous pulse trains. Typically, a phase-locked loop includes a phase detector, a lowpass filter, and a voltage controlled oscillator (VCO). The phase detector measures small differences in phase between the incoming signal and the feedback signal from the VCO. Any detected difference generates an error signal which is filtered (or averaged) by the lowpass filter. The filtered error signal is then applied to the VCO in such a way as to minimize the difference signal.
In digital transmission systems tight frequency control and short acquisition times are required for optimum system operation. Unfortunately these two features are intrinsically antithetical. A reduction in acquisition time involves an increase in the loop bandwith, and a wide loop bandwith implies little or no control over the VCO. In digital timing circuits, this is particularly deleterious since a wide loop bandwith results in high output phase jitter.
Several techniques of acquisition have been suggested to find an optimal compromise between these two requirements. None of them has given truly satisfactory results. See Phase Locked Techniques, by Floyd M. Gardner, Section 4--4, "Techniques of Acquisition", John Wyley and Sons, Inc. 1966. One such known technique employs a loop with two different bandwidths. For acquisition a wide bandwidth is used and for tracking a considerably narrower one is used. To change bandwidths only resistors, i.e. non-energy storage components, have been used since an unchanged capacitor or inductor switched back into a filter would likely disturb the filter voltages and in the process upset the loop lock. This restriction on filter design does not provide completely satisfactory phase-locked loop results. Only limited changes in bandwith can be obtained from resistive component changes. The invention herein discloses a method which overcomes this phase-locked loop design restriction.
Accordingly it is a broad object of the present invention to improve the acquisition period of a phase-locked loop having tight bandwidth control.
More particularly it is an object of this invention to provide a method of switching out during the acquisition period certain filter elements including capacitors, and then switching these filter elements back into the filter circuit for narrow bandwidth control.